A range of implanted neural devices exist, including: spinal cord implants which electrically stimulate the spinal column in order to suppress chronic pain; cochlear implants which electrically stimulate the auditory nerve to produce a hearing sensation; deep brain stimulators which electrically stimulate selected regions of the brain to treat conditions such as Parkinson's disease or epilepsy; and neural bypass devices which electrically stimulate either afferent sensory nerve fibres to reproduce impaired sensory function or efferent motor nerve fibres to reproduce impaired motor activity, or both.
Such devices require implantation of an electrode array proximal to the neural pathway of interest, in order to enable electrical stimuli to be delivered from the array to the nerve in order to evoke compound action potentials, or neural responses. For example, the typical procedure for implantation of a spinal cord stimulator having a paddle electrode involves placing the patient under general anaesthesia and performing a laminectomy or removal of part of the dorsal process to access the epidural space. However the success of spinal cord stimulation for pain relief, and of neural device implantation in general, is strongly linked to the accuracy of the placement of the implanted stimulating electrodes during surgery. Physiologic midline placement of paddle leads is important to avoid uncomfortable side-effects during stimulation as a result of the activation of dorsal root fibers. One approach to accurately position the electrode array is to temporarily wake the patient from the general anaesthesia and to ask the patient to report the location of paraesthesia produced by stimuli delivered by the array. Temporarily waking a patient from a general can be difficult, and even once the patient is awake the reports provided by a drowsy patient are often unreliable. Because the patient is not fully alert when temporarily awoken from general anaesthesia, and is otherwise asleep during the remainder of the implantation procedure, they can only provide limited feedback regarding the location of the paraesthesia, or regarding any complications arising from lead placement. Although complications are rare they can be very serious.
Another option is to not wake the patient during surgery, and to use anatomical targeting to guide the positioning of the electrode array, by reference to anatomical markers that can be imaged via fluoroscopy, instead of relying on unreliable patient feedback. However, fluoroscopic imaging resolution is relatively imprecise, compared to the accuracy requirements of lead placement. Moreover, complications of implanting a surgical lead while a patient is asleep can include damage to the spinal cord due to direct pressure of the lead as it is placed into the epidural space, or post-operative damage due to the development of a hematoma over the lead, which can then create pressure on the lead and damage the dorsal column axons.
Another situation requiring accurate electrode lead placement is the case of paddle leads, which comprise a two dimensional array of electrodes which when implanted into the epidural space extend both along (caudorostrally relative to) and across (mediolaterally relative to) the dorsal columns. Paddle leads for example can be used to treat patients with bilateral pain complaints, with the goal to provide paraesthesia to both sides of the body. To accomplish this it is preferable to place the paddle lead over the physiologic midline of the dorsal columns. However the physiologic midline, being the centre line of the spinal cord which demarcates between the fibres innervating the left side and the right side of the body, may or may not be well aligned with the anatomical midline as defined by anatomical markers that can be imaged via fluoroscopy. Consequently, implanting a patient under a general anaesthetic by reference to anatomical markers can result in the paddle electrode array not providing equal stimulation and paraesthesia to both sides of the body.
One technique for defining the physiologic midline is to use somatosensory potentials, observed from electrodes placed on the scalp. In this technique the stimulation of peripheral nerve fibres, such as stimulation of the posterior tibial nerve by needle electrode, evokes a response in the somatosensory cortex. By simultaneously stimulating dorsal column fibres using the spinal cord lead, a collision can be created between the peripherally evoked response and the spinally evoked response. This collision results in an observed depression of the somatosensory responses. Both tibial nerves are stimulated, so that a symmetric depression from left and right somatosensory cortex responses will indicate that the stimulated electrode is above the midline.
Somatosensory response to stimulation of peripheral nerves has also been used to identify the rostral caudal location of the electrode with respect to peripheral locations. However, this has been less successful as when considering a sensory homunculus the representation of the legs for example on the sensory cortex is small, and buried within the longitudinal fissure of the brain. Since many chronic pain patients have lower extremity pain this method has not proved to be useful. Another method has been to record motor evoked potentials from the muscles in the periphery in response to stimulation at the spinal cord. Although more successful at activating muscle fibres, dorsal column motor stimulation requires very high currents and as such does not closely correspond to the area of sensory activation.
The dorsoventral position of the electrode array is also of importance, as a large nerve-to-electrode distance can increase the stimulus power required to evoke neural responses and thus decrease battery life. A large electrode-to-nerve distance can also decrease the strength of observed neural signals reaching sense electrodes, in devices configured to measure the neural responses. On the other hand, bringing the electrode array too close to the nerve can apply pressure or direct trauma to the nerve and cause temporary or even permanent nerve damage. However, the dorsoventral position is also difficult to accurately determine during surgery. Occasionally a surgeon may take a lateral view image with fluoroscope, however these images are not of sufficient resolution to sufficiently accurately judge the proximity of the array to the cord.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
In this specification, a statement that an element may be “at least one of” a list of options is to be understood that the element may be any one of the listed options, or may be any combination of two or more of the listed options.